1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to an active matrix-type liquid crystal display device having a storage capacitor.
2. Description of the Related Art
An active matrix-type liquid crystal display device having a switching element for each of pixel electrodes, which provides a highly precise and high density display, has been put into practical use, and further actively researched and developed for creating a highly precise, high density and large screen display.
As shown in FIG. 2, an active matrix display device comprises a gate driver 47 and a source driver 48, a plurality of gate bus lines 41 (G1-Gn) and source bus lines 42 (S1-Sm) each of which crosses at right angle, a thin film transistor (TFT) substrate having a thin film transistor 43 and a pixel electrode 44 which are disposed in a cross portion of each of these bus lines, a liquid crystal interposed between the TFT substrate and its opposite substrate, and a liquid cell 46 formed between the pixel electrode 44 and a common electrode 45 of the opposite substrate. The gate electrode and the source electrode of the thin film transistor 43 are connected to each of the gate bus lines 41 and the source bus lines 42, respectively, and the pixel electrode 44 is connected to a drain electrode. While a scanning signal is successively applied from the gate driver 47 on each of the gate bus lines 41, an image signal from the source driver 48 is supplied and written into the individual liquid crystal cell 46 through the thin film transistor 43. Then, the potential of the pixel electrode 44 is held in a capacitor of the liquid crystal cell 46 until the image signal is rewritten in the next frame period.
However, the holding potential creates a level shift toward the negative electrode side due to the charge redistribution in a capacitor formed between the gate electrode and the drain electrode, since the potential applied to a liquid crystal cell is asymmetric between the negative and positive electrodes, and the direct current component is applied to the liquid crystal which is responsible for a display flicker. Also, the holding potential of the pixel electrode is gradually reduced due to a leak between the source and the drain in the thin film transistor and a self-discharge through the resistance component of the liquid crystal cell during the holding period. These problems become more prominent with the development of a highly precise liquid crystal display device and a finer pixel electrode.
In order to solve the aforementioned problems, an improved liquid crystal display device has been proposed which comprises a storage capacitor placed parallel to the liquid crystal cell to control the level shift due to the charge redistribution and at the same time provides an increased pixel capacity as well as an increased discharge time constant to prevent the reduction of the holding potential. This storage capacitor has either a structure comprising the partially overlapped previous gate bus line and pixel electrode, or a structure comprising a capacitor formed between a separately formed auxiliary electrode line and a pixel electrode. The former provides a wider aperture ratio, but is affected by the variation in the potential applied on the gate bus line. Although the latter provides a stable pixel potential, its lowered aperture ratio provides a reduced display brightness. At present, a liquid crystal display device with the latter structure has often been used due to its stable display screen. The liquid crystal display device having such a structure include those described below.
Japanese Laid-open Patent Publication No. 5-289111 discloses a liquid crystal display device comprising a pixel electrode, a passivation film formed on the pixel electrode, and a storage capacitor electrode formed on the passivation film. However, this liquid crystal display device suffers from the reduction of the potential applied to the liquid crystal layer.
Japanese Laid-open Patent Publication No. 5-216067 discloses a liquid crystal display device comprising two insulating layers formed between a gate electrode and a pixel electrode. The liquid crystal display device comprises a thin film transistor array substrate, an opposite substrate having a common electrode, and a liquid crystal layer interposed between these substrates. As shown in FIGS. 3 and 4, the thin film transistor array substrate comprises a transparent substrate 51; a silicon semiconductor layer 52 formed thereon; a n-type or p-type impurity-doped polysilicon region 61; and a gate insulating film 53, a gate electrode 54, a source electrode 55 and a drain electrode 56 which are successively formed on the silicon semiconductor layer 52; a pixel electrode 57 connected to the drain electrode 56; two insulating layers 58 and 60 disposed between the gate electrode 54 and the pixel electrode 57; and a transparent storage electrode 59 formed on the first insulating layer 58. Thus, a storage capacitor is formed in an overlapping portion of the transparent storage electrode 59 and the pixel electrode 57. In such a structure, since the transparent storage electrode 59 is made from a transparent material, the resulting liquid crystal display device provides an improved aperture ratio.
However, in the case of the structure shown in FIGS. 3 and 4, the transparent storage electrode 59 is formed by high temperature processing at about 1000.degree. C. and the first insulating layer 58 thereunder is also heated to about 1000.degree. C. Therefore, the first insulating layer 58 is easily destroyed. And, when the first insulating layer 58 of the liquid crystal display device has a higher dielectric constant, the parasitic capacity of the bus lines will be increased. On the other hand, when the second insulted layer 60 has a lower dielectric constant, the storage capacity will not be sufficiently increased. For that reason, it is necessary to extend the area of the transparent storage electrode 59 so as to increase the storage capacity. Also, because the transparent storage electrode 59 is placed in the pixel region of the liquid crystal display device, a certain amount of light absorbed into the electrode may cause the reduction of the light transmittance in the device. Moreover, where both the transparent storage electrode 59 and the pixel electrode 57 are made from the same material such as indium tin oxide (ITO) or the like, any defect of the second insulating layer 60 may cause the generation of a short-circuit between the transparent storage electrode 59 and the pixel electrode 57.